The present invention relates to a method and device for the braking of vehicles through the servo-control of the braking moment or braking torque applied to a wheel. It can be applied notably to the braking systems of all vehicles where the braking has to be done in very safe conditions.
The presently known braking control devices mainly use, as their basic parameter, the measurement of the rotational speed of the wheels. By means of this parameter and the characteristic equation of the braking, they monitor the variations in the acceleration and deceleration of the vehicle. Furthermore other devices implement a speed servo-control system to monitor the speed of the braked zone which, when compared with the reference speed of the vehicle, determines maintaining of the braked wheel at a fixed wheel slip value, the slip value being the difference between the speed of the vehicle and the rotational speed of the wheel multiplied by its radius.
In the above two cases, the performance of the device is directly related to the number of measurement signals per wheel rotation delivered by the sensor mounted on the wheel to be controlled. Depending on these applications, this number may vary from 50 to 200 signals per wheel rotation, which necessitates sophisticated sensors that are costly and not sufficiently reliable. These methods have other drawbacks. In particular, it is difficult to detect the deceleration of the speed of the wheel when the curves representing the brake friction coefficient as a function of slip have a low maximum point or are flat, like the curves encountered when the ground is wet, for example. In fact, these methods do not enable optimizing of the stopping distance but instead prevent the wheels from becoming locked. The braking efficiency of the systems made according to these methods is about 60% to 70%. This braking efficiency is defined by the ratio between the real braking distance obtained and the theoretical braking distance computed for a maximum value of a given curve of values or a given adherence curve. Finally, these methods have the major drawback of not providing a high degree of safety in the braking of vehicles. Indeed, the operations of detection or regulation are carried out based on the rotational speed of the wheels without the values of the braking torque exerted on these wheels being taken into account. Now, for dictated or instructed values of speed that are identical, the values of braking torque applied to the wheels are generally different. This creates braking dissymmetries that could result notably in loss of control of the direction of travel of the vehicles.
In order to avoid such accidents, there are methods wherein the braking is controlled by means of two systems, firstly a speed regulation loop and secondly a computer. The regulation loop provides for the fast correction of the system when there are disturbances while the computer, using information based on the speed of the braked wheel and on braking torque, computes the curve of the values of adherence between the tire and the track, determines the maximum point of this curve and prepares a signal which, in modifying the instructed value of the speed regulator, leads the system to this maximum point. While the speed regulator makes fast comparisons of divergence from the instructed slip value, the computer carries out an operation of pre-regulation by taking account of the trends or slow drifts and modifies the instructed value of the regulator in order to bring operation of the system close to the maximum point of the adherence curve and keep it there. However, these methods imply linear relationships, firstly between the pressure exerted on the brake and the control current delivered by the regulator and, secondly, between the braking torque exerted on the wheel and the pressure exerted on the brake. In reality, no such linear relationships exist, for they are in fact only very approximate relationships. To overcome this drawback, there are approaches proposing a direct servo-control of the braking torque exerted on each of the wheels of the vehicle. These approaches, which are satisfactory in theory, are however very difficult to implement notably because they require the use of very sophisticated and very costly torque sensors and speed sensors in order to be made efficient.